1. Technical Field
The present invention relates to a corner supply type plane light source device making use of optical means having both light guide function and scattering function in an internal volume region (hereinafter called light-scattering light guide). The plane light source device of the invention is applied to backlight source means of display device such as liquid crystal display.
2. Description of the Related Art
In a known plane light source device of a thin panel type used in backlight source means of various display devices including liquid crystal display, a light guide of straight tube type (typically, a fluorescent lamp) is disposed at a position opposite to the side end face of a plane light guide, and light is supplied to the light guide from the straight tube light source. The light introduced into the light guide is converted in propagation direction, and is emitted from a light exit surface (luminescent surface). In such plane light source device, to form a luminescent surface of a relatively wide area on the surface of the light guide, light supply means of a large size must be disposed at least in one direction. This is not, however, an advantageous condition for saving power in the plane light source device or simplifying the structure.
On other hand, in the plane light source device having a flat light guide, a technique for disposing light supply means at a corner or corners of the light guide has been proposed, but it is difficult to distribute the light entering from a corner of the light guide uniformly in the entire light guide, and practical plane light source device has not been realized yet. A light-emitting diode (LED) of high luminance and small size has been developed, and its size is considered to be suited to be disposed in the corner of the light guide. However, when an LED between a type with strong directivity in radiation light is used for enhancing the efficiency of light coupling between the LED and the light guide, a plane light source device high in uniformity of brightness is not realized in the constitution of disposing at the corner of the light source.
To avoid this problem, when high luminance LEDs are used as light supply means of plane light source device, a light diffusion plate is combined by arranging the LEDs in a two-dimensional array. Therefore, if the light supply means of high luminance LEDs is used, the entire structure including the wiring is complicated, and it is difficult to save power and simplify the structure.
Moreover, in the conventional plane light source device for obtaining an illumination light by converting the propagation direction of incident light by using light scattering in the light-scattering light guide, a considerable portion of the light entering in the light-scattering medium is scattered backward, and tends to return to the light source side without being guided sufficiently in the scattering medium. Such return light not only causes light loss, but also raises locally the luminance in the portion near the light supply source and generally lowers the luminance in the portion remote from the light supply source.